Feature: Core Animation
How it works: Core Animation is a new framework in Leopard. This framework makes it easy for developers to harness the graphics power in OS X for their own applications. Using layers, a developer can animate a number of different objects of different types (including video, images, and OpenGL renderings) and apply different effects to each. Core Animation does the heavy lifting of actually figuring out what needs to happen in each frame (developers set the start, the end, and some key frames and Core Animation does the rest). Core Animation is going to usher in some astounding new UI on the Mac (Time Machine is a great example of what is to come).
Who will use it: Developers will use it directly, but only who uses apps built on the Core Animation framework will benefit from it.